The present invention relates to a method of manufacturing a solid electrolytic capacitor and, more particularly, to a method of manufacturing a solid electrolytic capacitor using a conductive polymer as a solid electrolyte.
Electrolytic capacitors using a valve action metal such as tantalum or aluminum are popularly used. As a characteristic feature of such an electrolytic capacitor, the surface area of a dielectric layer is increased (surface extension) in the form of a sintered body or an etching foil, so a large capacitance can be obtained in a small size. However, since an electrolyte of manganese dioxide or ethylene glycol is used, the impedance becomes high at a high frequency.
Along with recent progress in compact and high-performance electronic devices, high-frequency and digital electrical circuits also make an advance, and demand for capacitors having excellent high-frequency characteristics has arisen. To meet this requirement, solid electrolytic capacitors using, as a solid electrolyte, a conductive polymer having a conductivity several hundred times higher than that of a conventional electrolyte have been proposed. As the electrolyte of such a solid electrolytic capacitor, a pentacyclic compound as a conductive polymer such as polypyrrole is doped with a dopant to attain a conductivity and used, so the solid electrolytic capacitor has more excellent frequency characteristics than those of a conventional electrolytic capacitor.
This electrolytic capacitor has excellent high-frequency characteristics because of the high conductivity of the electrolyte. In addition, since no thermal hysteresis is added to form the electrolyte, the oxide film is not damaged. Therefore, the reliability is higher than an electrolytic capacitor using a thermal decomposition product such as manganese dioxide as an electrolyte.
To form a conductive polymer on an oxide film, chemical oxidation polymerization or electrolytic oxidation polymerization is mainly used.
In electrolytic oxidation polymerization, no charges can be present on the oxide film as an insulator. For this reason, a precoating layer of a conductive polymer or manganese dioxide is formed on the oxide film by chemical oxidation polymerization, and then, a conductive polymer is formed by electrolytic oxidation polymerization.
However, when the conductive polymer is formed in a sintered body or etching pit by electrolytic oxidation polymerization, the field strength in pores readily differs from that outside the pores. For this reason, the conductive polymer preferentially forms outside the pores with a high field strength, so the capacitor cannot obtain a sufficient coverage. In addition, electrolytic oxidation polymerization requires to control the current value in units of capacitor elements and therefore is not convenient for industrial production.
On the other hand, chemical oxidation polymerization allows batch processing of a lot of capacitor elements and is relatively convenient for industrial production. In recent years, extensive studies have been made for a technique of forming a conductive polymer by chemical oxidation polymerization, and the following reports are presented.
(1) U.S. Pat. No. 4,697,001 (reference 1) discloses polymerization of pyrrole using ferric dodecylbenzenesulfonate or the like as an oxidant.
(2) "Tantal Solid Electrolyte Capacitor with Polypyrrole Electrolyte Prepared by Chemical Polymerization using Aqueous Solution", DENKI KAGAKU, Vol. 64, No. 1, pp. 41-46, January 1996 (reference 2) discloses chemical oxidation polymerization using an aqueous solution containing a pyrrole monomer and a surfactant (sodium alkylnaphthalenesulfonate) and an aqueous solution of an oxidant containing ferric sulfate and a surfactant (sodium alkylnaphthalenesulfonate). In this technique, a sintered body is dipped in the aqueous solution containing the pyrrole monomer and then dipped in the aqueous solution containing ferric sulfate as an oxidant, and this operation is repeated to form a polypyrrole layer.
(3) Japanese Patent Laid-Open No. 8-45790 (reference 3) and Japanese Patent Laid-Open No. 7-70294 (reference 4) disclose chemical oxidation polymerization using a polymerization solution containing a conductive polymeric monomer, an oxidant, and water. In this technique, 2 wt % or more of water are added to a solvent for dissolving both the conductive polymeric monomer and the oxidant.
(4) Chemical oxidation polymerization in which step S21 of dipping a sintered body in the monomer solution, step S22 of drying and polymerizing the monomer solution, step S23 of dipping the sintered body in the oxidant solution, and step S24 of drying and polymerizing the oxidant solution, as shown in FIG. 5, are performed in this sequence is disclosed.
In polymerization disclosed in reference 2 as well, after the step of dipping the sintered body in the monomer solution, the step of dipping the sintered body in the oxidant solution is performed.
However, the above-described prior arts have the following problems.
(1) When a conductive polymer layer is formed by chemical oxidation polymerization disclosed in reference 2 or 3, the capacitor cannot obtain a sufficient coverage, so the leakage current considerably increases in mounting the capacitor on a circuit board.
The reason will be described with reference to FIGS. 4A, 4B, and 6A to 6D. FIG. 4A schematically shows a solid electrolytic capacitor having a conductive polymer formed by the prior art disclosed in reference 2 or 3, and assembled by a known technique. FIG. 4B shows the main part of FIG. 4A.
As shown in FIG. 4B, in the conventional solid electrolytic capacitor, an oxide film 3, a conductive polymer layer 4, a carbon layer 5, and a silver paste layer 6 are sequentially formed on the surface of a tantalum sintered body 1 shown in FIG. 4A, in which a anode lead 2 is implanted. Subsequently, external electrode terminals 7 and 8 are extracted, and the entire structure is molded with an epoxy resin 10, thereby completing the solid electrolytic capacitor.
FIG. 4B schematically shows a state wherein the conductive polymer layer 4 is not sufficiently formed in the capacitor element (sintered body 1), and the conductive polymer layer 4 on the surface of the capacitor element has no sufficient thickness. Reference numeral 9 denotes a conductive adhesive 9; and 15, a gap.
FIGS. 6A to 6D schematically show the capacitor element surface so as to explain the steps in forming the conductive polymer layer in FIG. 5.
FIG. 6A shows a portion near the surface of the oxide film 3 in the conductive polymeric monomer solution dipping step (step S21). FIG. 6A schematically shows a state wherein pores of the tantalum sintered body 1 are sufficiently filled with a conductive polymeric monomer 13a and a conductive polymeric monomer solvent 14a.
FIG. 6B shows the portion near the surface of the oxide film 3 in the conductive polymeric monomer solution drying step (step S22). When the conductive polymeric monomer 13a is a liquid such as pyrrole with a high vapor pressure, evaporation of the conductive polymeric monomer 13a in the pores is conspicuous. FIG. 6B shows a state wherein the conductive polymeric monomer 13a and the conductive polymeric monomer solvent 14a held on the capacitor element surface and in the pores evaporate and decrease.
FIG. 6C shows the portion near the surface of the oxide film 3 in the oxidant solution dipping step (step S23). The pores are filled with an oxidant 11 and an oxidant solvent 12, so the oxidant 11 contacts the conductive polymeric monomer 13a and polymerizes it.
In the conventional combination of reactive solutions, e.g., in the combination of the aqueous solution of a pyrrole monomer and the aqueous solution of ferric sulfate, both solutions use water as a solvent, and the pyrrole 13a as a conductive polymeric monomer held in the pores is readily diffused. For this reason, contact between the pyrrole 13a and the oxidant 11 tends to occur at a portion far from the oxide film surface, and the conductive polymer layer cannot be sufficiently formed on the oxide film 3 in the pores, resulting in a low coverage.
Additionally, the pyrrole 13a diffused in the pores moves toward the element surface and preferentially reacts, near the surface, with the oxidant 11 entering from the element surface, so polypyrrole is preferentially formed near the pore inlet on the surface. Normally, the conductive polymer layer of polypyrrole is preferentially formed near the pore inlet on the capacitor element surface at the early stage of the polymerization cycle because the sintered body is repeatedly dipped in the conductive polymeric monomer solution and the oxidant solution a plurality of number of times.
After this, since the conductive polymeric monomer or oxidant does not enter the pores, the conductive polymer layer 4 is not uniformly formed on the oxide film in the pores, resulting in a low coverage, as shown in FIG. 4B.
Diffusion of pyrrole 13a also occurs on the capacitor element surface, as a matter of course, so the pyrrole 13a is diffused from the capacitor element surface. No polypyrrole is formed on the element surface where the pyrrole 13a is absent, so the polypyrrole film thickness on the element surface becomes nonuniform. Especially, if the number of times of polymerization is small, the oxide film 3 has no polypyrrole at some portions on its surface microscopically.
In mounting the capacitor on a circuit board, thermal stress of the molding resin may be applied to the oxide film 3 to damage the oxide film 3, resulting in an increase in leakage current. When the polypyrrole layer is formed on the oxide film 3, the oxide film 3 locally generates heat as the leakage current increases. The polypyrrole layer formed on the oxide film is thermally oxidized to lose the conductivity. As a result, the current to the defective portion of the oxide film 3 is blocked. That is, even when the leakage current temporarily increases, the current returns to the normal level (insulation restoration function).
However, if no polypyrrole layer is formed on the oxide film 3, the above-described insulation restoration does not take place, as a matter of course, so the increase in leakage current poses a problem. In addition, when the polypyrrole layer formed on the oxide film 3 is thin, the leakage current increases after the capacitor is mounted on the circuit board. Therefore, a sufficient film thickness must be ensured.
The reason why the leakage current increases in mounting the capacitor on the circuit board when the polypyrrole layer is thin is not yet clarified. As a probable reason, a shift is caused in the polypyrrole layer due to thermal stress in mounting the capacitor on the circuit board, and microscopically, some portions of the oxide film have no polypyrrole layer, and the leakage current increases due to the above-described reason.
On the other hand, when the polypyrrole layer is thick, the problem of leakage current in mounting the capacitor on the circuit board can be solved, though the ESR (Equivalent Series Resistance) of the capacitor increases. Conventionally, since the thickness of the polypyrrole layer readily becomes nonuniform, it is difficult to control the film thickness within an appropriate range.
FIG. 6D shows the portion near the surface of the oxide film in the oxidant solution drying step (step S24). The pyrrole 13a and the oxidant 11, which are held in the pores without being diffused into the oxidant solution in the oxidant solution dipping step (step S23) react with each other to form polypyrrole.
For polymerization, steps S21 to S42 in FIG. 5 are repeated a plurality of number of times. After completion of step S24, if an excess oxidant remains on the capacitor element, the residual oxidant 11 reacts with the pyrrole 13a in the subsequent pyrrole solution dipping step (step S21) to form polypyrrole. In this case as well, the oxidant 11 remaining on the capacitor element is eluted and diffused into the pyrrole solution. For this reason, the conductive polymer layer having a sufficient coverage cannot be formed in the pores, and additionally, a sufficient thickness can hardly be ensured on the capacitor element surface.
The reason for this is as follows. In the pyrrole monomer solution dipping step, ferric sulfate held in the pores in advance is eluted and diffused into the aqueous solution of pyrrole because the ferric sulfate is soluble in water.
Since the technique disclosed in reference 3 also uses a solvent for dissolving both the conductive polymeric monomer and the oxidant, the conductive polymeric monomer held in the pores in advance is eluted and diffused into the oxidant solution. For this reason, conductive polymer formation as an object hardly occurs in the capacitor element. In addition, reaction between the oxidant and the conductive polymeric monomer preferentially takes place on the surface of the capacitor element, so the capacitor cannot obtain a sufficient coverage.
As described above, in the prior art disclosed in reference 3, the oxidant is eluted and dissolved into the conductive polymeric monomer solution, and polymerization cannot be efficiently performed on the capacitor element.
(2) As the disadvantage of the technique disclosed in reference 4, the capacitor cannot obtain a sufficient coverage. The reason is as follows. Since the polymerization solution is held at a low temperature of -30.degree. C. or less, the viscosity of the polymerization solution is high, and the pores in the sintered body cannot be sufficiently filled with the polymerization solution.
(3) As the disadvantage of the technique associated with the sequence shown in FIG. 5, the capacitor cannot obtain a sufficient coverage. The reason is as follows. When the conductive polymeric monomer such as pyrrole is a liquid at room temperature and humidity, the conductive polymeric monomer evaporates together with the solvent in drying the conductive polymeric monomer solution (step S22 and FIG. 6B). Especially, for pyrrole having a high vapor pressure, the amount of the remaining conductive polymeric monomer can hardly be controlled to a predetermined amount before oxidant solution dipping, and the conductive polymer formation amount varies.